Lyme disease is a multifaceted illness, initiated upon infection with the spirochete Borrelia burgdorferi (Bb). One manifestation of the disease is arthritis that can become a debilitating, chronic disease in genetically susceptible individuals. During the tenure of this grant, a candidate auto-antigen, hLFA-1, was identified which may elicit an autoimmune response in T cells by molecular mimicry with outer surface protein A (OspA) of Bb. These data form the basis for the current proposal. I. The analysis of the inflammatory T cell response will be expanded and refined by: a) single cell sorting of OspA-reactive T cells from Lyme arthritis patients, using cytokine secretion/capture assays to identify and clone OspA-reactive T cells, regardless of epitope fine specificity. The T cell response of patients in the acute and the chronic phase of the disease will be compared that will provide insights into the mechanism of treatment resistant Lyme arthritis. b) Analyses of OspA responses in HLA.DRB1 transgenic mice. DRB1*0401 and *0101 are associated with susceptibility to chronic Lyme arthritis, while the presence of *1101 seems to protect from this disease. To study the underlying mechanism, the OspA immune response in Bb infected C3H mice expressing these human DRB chains will be mapped and compared. c) testing hLFA-1/DRIB 1*0401 transgenic mice as an animal model for chronic Lyme arthritis. These mice will be infected with Bb and screened for the development of a chronic arthritic disease. II. The contribution of Abs to the inflammatory joint disease will be assessed. A prominent OspA-specific Ab response is observed during the chronic phase of the disease, correlating with the onset of prolonged bouts of arthritis. These Abs will be tested for an autoimmune reaction, and their involvement in joint inflammation will be determined by: a) the identification of immune complexes in synovial lesions of Lyme arthritis patients; b) the use of OspA Abs from chronic Lyme arthritis patients as probe for autoantigen; c) Bb infection in FcRn-deficient, hLFA-1/ DR*0401 transgenic mice: FcRn-/- mice clear serum IgG fast and are protected from Ab-mediated autoimmune diseases. III. A new candidate for molecular mimicry of OspA, hMAWD-BP, will a) be tested for cross reaction with OspA specific T cells and Abs, b) its aa sequences in the OspA-homologous regions of human and mouse will be compared; c) if significant differences are observed, hMAWD-BP transgenic mice will be prepared; finally, d) Genebank searches will be carried out for the identification of other human homologues of bacterial OspA. These experiments are designed to elucidate the mechanism of treatment resistant Lyme arthritis.